题目内容：
根据下面资料，回答题 Volts From the Sky
①Lightning has caused awe and wonder since old times. Although Benjamin Franklin demonstrated lightning as enormous electrical discharge more than 200 years ago, many puzzles still surround this powerful phenomenon.
②Lightning is generated when electrical charges separate in rain clouds,though processes arestill not fully understood. Typically, positive charges build at the cloud top, while the bottom becomesnegatively charged. In most instances of cloud-to-ground lightning ,the negatively charged lower portion
of the cloud repels negatively charged particles on the ground' s surfaces, making it become positivelycharged. The positive charge on the ground gathers at elevated points. A flow of electrons beginsbetween the cloud and earth. When the voltage charge becomes large enough, it breaks through theinsulating barrier of air, and electrons zigzag earthward. We see the discharge as lightning.
③Lightning can occur within a cloud, between clouds,or between clouds and the ground. The firstvariety,intra-cloud lightning,is the most frequent but is often hidden from our view. Cloud-to-groundlightning, making up about 20 percent of lightning discharges, is what we usually see. Lightningcomes in several forms, including sheet, ribbon, and ball. Intra-cloud lightning can illuminate a cloud
so it looks like a white sheet, hence its name. When cloud-to-ground lightning occurs during strongwinds, they can shift the lightning channel sideways, so it looks like a ribbon. The average lightningstrike is more than 3 miles long and can travel at a tenth of the speed of light. Ball lightning, therarest and most mysterious form, derives its name from the small luminous ball that appears near the
impact point, moves horizontally, and lasts for several seconds.
④Thunder is generated by the tremendous heat released in a lightning discharge. Temperaturesnear the discharge can reach as high as 50,000°F within thousandths of a second. This suddenheating acts as an explosion, generating shock waves we hear as thunder.
⑤About 2,000 thunderstorms are occurring in the world at any tie, generating about 100lighting strikes every second, or 8 million daily. Within the United States, lightning strikes are estimated at 20 million a year, or about 22,000 per day. You have a 1-in-600,000 chance of beingstruck by lightning during your lifetime. Lightning can strike twice or more in the same spot. The Empire State Building in New York is struck by lightning about two dozen times annually.
In most cases of cloud-to-ground lightning, the ground' s surface__________
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